This invention is in the field of electronic circuits and systems, and is more specifically directed to feedback control in such circuits and systems.
Many modern electronic systems involve circuits that generate an output voltage or current, and feedback control of that output. A common example of such a circuit is a switching power supply, which generates a stable power supply voltage over varying loads, typically by controlling a push-pull output driver with a pulse-width modulated (PWM) control signal. Conventional switching power supply subsystems include a power supply controller, which receives a signal corresponding to the currently generated output voltage, and which adjusts the duty cycle of the PWM control signal in response to that output voltage. Other feedback-controlled electronic circuits and systems include motor controllers and other control systems.
In conventional switching power supplies, by way of example, power supply control circuits are typically implemented in the analog domain. The typical conventional analog controller involves an operational amplifier (“op amp”) that generates an analog voltage in response to an analog comparison between a measure of the output signal and a reference voltage. The analog op amp output is applied to circuitry that generates PWM pulses at a duty cycle corresponding to the comparison; these pulses are applied to the gates of push-pull drive transistors. The duty cycle of the PWM pulses determines the drive applied by the push-pull drive transistors, with the duty cycle increased if the output voltage is too low and the duty cycle decreased if the output voltage is too high. In this conventional manner, the power supply output voltage is regulated to the desired voltage.
Analog power supply controllers have several drawbacks, however. As known in the art, analog control circuitry is generally quite vulnerable to noise. In addition, analog feedback controllers generally require discrete components, including passive components, which add to manufacturing cost and also occupy relatively large printed circuit board area. These discrete components are also often of such size and characteristics that integration into a single integrated circuit (or single “chip”) is not possible for analog feedback controllers, particularly for high power output such as power supply control. And perhaps most importantly, analog power supply controllers are inflexible once implemented, typically requiring a complete redesign if the characteristics of the controller are to be modified.
It is therefore desirable to use digital circuitry for feedback controllers, such as switching power supply controllers. If digital control were available, modifications to software or firmware could effect modifications to the overall controller function, without requiring redesign of the controller and certainly without requiring replacement of components in the function. To the extent that this digital control is programmable, additional and different functions may be added to or otherwise implemented in the feedback controller, enabling added value to the overall system. In addition, proprietary feedback controller designs can be protected to a much higher degree in digital controllers, particularly those operating according to program code stored in hardware.
It has been observed, however, that conventional digital feedback controllers are prohibitively expensive for many applications. As known in the art, digital feedback control typically involves digital filter functions, of either or both of the infinite impulse response (IIR) or finite impulse response (FIR) type. These digital filters, as known in the art, typically involve multiply-and-accumulate operations. In order to perform these operations at the desired rate, conventional digital feedback controllers use a digital signal processors (DSPs). Examples of conventional DSPs are the TMS320c5X and TMS320c6X families of digital signal processors, available from Texas Instruments Incorporated. While these DSP devices have excellent performance and are well able to perform this control function, a DSP solution for this application is much more expensive than a corresponding analog controller. Less-expensive digital functions, such as microcontrollers, do not have the computational performance and capacity to perform real-time control.